Equinatoxin, a highly basic protein extracted from Actinia equina, causes an increase in the survival time of mice bearing the ascitic form of Ehrlich carcinoma, whereas it has no effect on L1210 leukaemia. When tested for in vitro cytotoxicity by the dye exclusion test, it shows a potent activity on both tumour cell lines, with ED50 of a few ng/ml. Higher concentrations produce an extensive lysis of the cells. The cytotoxic effects of Equinatoxin are inhibited by phospholipids, thus suggesting that its mechanism of action may be related to interactions with lipids or other charged components of cell membrane. The observed lack of in vivo activity against L1210 leukaemia presumably is due to poor systemic absorption of the protein and/or neutralization by serum factors.